It is by now axiomatic that the phenotype of an organism is largely determined by the genes expressed within it. These expressed genes can be represented by a “transcriptome,” conveying the identity of each expressed gene and its level of expression for a defined population of cells. Unlike the genome, which is essentially a static entity, the transcriptome can be modulated by both external and internal factors. The transcriptome thereby serves as a dynamic link between an organism's genome and its physical characteristics.
The transcriptome as defined above has not been characterized in any eukaryotic or prokaryotic organism, largely because of technological limitations. However, some general features of gene expression patterns were elucidated two decades ago through RNA-DNA hybridization measurements (Bishop et al., 1974; Hereford and Rosbash, 1977). In many organisms, it was thus found that at least three classes of transcripts could be identified, with either high, medium, or low levels of expression, and the number of transcripts per cell were estimated (Lewin, 1980). These data of course provided little information about the specific genes that were members of each class. Data on the expression levels of individual genes have accumulated as new genes were discovered. However, in only a few instances have the absolute levels of expression of particular genes been measured and compared to other genes in the same cell type.
Description of any cell's transcriptome would therefore provide new information useful for understanding numerous aspects of cell biology and biochemistry.